The Carbon Cycle 3

1
The Carbon Cycle 3

1. Introduction Changes to Global C Cycle (Ch. 15)
2. C-cycle overview pools fluxes (Ch. 6)
3. Controls on GPP (Ch. 5)
4. Controls on NPP (Ch. 6)
5. Controls on NEP (Ch. 6)

Powerpoint modified from Harte Hungate
(http//www2.for.nau.edu/courses/hart/for479/notes
.htm) and Chapin (http//www.faculty.uaf.edu/fffsc
/)
2

• IV. Controls on NPP
• A. Components of NPP
• B. Physiological Controls on NPP
• 1. Plant Respiration
• 2. Allocation
• C. State Factor and Interactive Controls on NPP
• 1. Climate
• 2. Parent material/nutrient availability
• 3. Organisms
• 4. Time
• V. Controls on NEP
• Measuring NEP
• Controls on NEP
• Uptake/release imbalance
• Disturbance

3

• NPP GPP - Rplant
• NPP DPlant/Dt Clost
• Clost exudates, vol. emissions, herbiv., tissue
  turnover (litterfall), disturbance (fire,
  harvest)
• NPP is total energy available to rest of ecosystem

6.8
4
Table 6.2 Components of NPP
Components of NPP of NPP New plant
biomass 40-70 Leaves and reproductive parts
(fine litterfall) 10-30 Apical stem
growth 0-10 Secondary stem
growth 0-30 New roots 30-40 Root
secretions 20-40 Root exudates 10-30 R
oot transfers to mycorrhizae 10-30 Losses to
herbivores, mortality, and fire 1-40 Volatile
emissions 0-5
What do we usually measure?? Litterfall Stem
growth Sometimes roots That leaves 30 or more
unaccounted for
5
What do we really care about?

• Biomass increment and carbon storage
• Energy available to other trophic levels
• Energy transfer to mycorrhizae (maybe)
• Root exudates (maybe)
• Volatile emissions (maybe important for atmos
  chem but less so for C accounting).

6
Controls on NPP Start Small or Big?
6.1
Plate 3
7
B. Physiological controls on NPP

• 1. Respiration
• NPP GPP Respiration

8
Respiration provides energy for essential plant
processes
Rplant Rgrowth Rmaint Rion
Respiration cost of growth is similar among
species and plant parts
9
b. What controls maintenance respiration?

• Plant chemistry
• especially protein content
• Environment
• especially temperature and drought

10
NPP is about half of GPP when looking across
biomes
6.6
11
2. Allocation (pp.132-135) Plants allocate most
growth to tissues that maximize capture of
limiting resources

• Allocate to roots when dry or nutrient poor
• Allocate to stems, leaves when light is limiting
• Constantly adjust allocation
• Prevents overwhelming limitation by any one
  resource
• Tends to make plants limited by multiple resources

12
C. State factor and interactive controls on NPP
1.3
13
NPP varies 14-fold among biomes
14
Biomass is greatest in tropical and temperate
forests
15
Half of global biomass and a third of global NPP
is in tropical forests (total area x
production/area)
16
C.1. Climate
17
Global patterns of NPP vary with
climate Increases with ppt (up to max at 2-3
m/yr) Increases exponentially with
temperature High variance due to variation in
other state factors
6.3
18
AET does better than temp or ppt alone.
What the heck is AET?
Molles 2004
19
PET responds mostly to changing temp (and
wind)AET is PET as constrained by available
precip.
PET decreases
AET decreases
2.21
20
PET responds mostly to changing temp (and
wind)AET is PET as constrained by available
precip.
PET same
AET decreases
2.21
21
AET does better than temp or ppt alone.
6.?
Molles 2004
22
NPP per unit leaf area and time is fairly similar
across biomes
23
Correlates with NPP
Leaf Area Index
3,000
2,500
2,000
NPP (g/m2/yr)
1,500
1,000
500
0
0
1
2
3
4
5
6
7
Leaf Area Index (m2/m2)
24
Length of the Growing Season
Correlates with NPP
3,000
2,500
2,000
NPP (g/m2/yr)
1,500
1,000
500
0
0
50
100
150
200
250
300
350
400
Growing Season Length (days)
25
does not correlate with NPP
Productivity per unit leaf area
3,000
2,500
2,000
Total Annual NPP (g/m2/yr)
1,500
1,000
500
0
0
1
2
3
Daily NPP per leaf area (g/m2/d)
26
This suggests that LAI and season length are
strong controllers of NPP as well as GPP
5.1
27
2. Parent material/soil resources

• a. Nutrient limitation of NPP
• Soil fertility gradients
• Fertilization

28
Which nutrients limit production? What do we mean
by limitation?
Simplistic view Liebigs law of the minimum
only one resource is limiting, that in most
limited supply But, multiple resource limitation
of NPP is frequently observed
29
Which nutrients?
Primary limitation, secondary limitation,
co-limitation
30
Why is NPP often limited by multiple resources?

• Adjustment of allocation to prevent overwhelming
  limitation by one resource
• Environment changes seasonally and from year to
  year
• Different resources limit different species

31
b. Climate effects are in part mediated by
belowground resources.
32
In ecosystems where correlations suggest a
strong climatic limitation of NPP
experiments and observations indicate that this
is mediated primarily by climatic effects on
belowground resources.
33
c. Interactive effects of nutrients vegetation
Soil/vegetation feedback

• Low nutrient environment

Low RGR, high CN, low biomass turnover
Slow decomposition
Low productivity
Slow mineralization
Chapin 1980
34
3. Organisms a. Vegetation composition
determines growth potential both across and
within biomes
35
b. Organisms x Climate interactions Direct
effects of climate on growth short-term temporal
variation Effects on species composition
spatial variation (which determines growth
potential) (takes time to adjust to climate)
6.5
36
4. Time Disturbance and succession are major
causes of variation in NPP within a biome
6.
37
V. Net ecosystem production (NEP)

• NEP GPP Recosyst
• Recosyst Rplant Rhet
• NEP NPP Rhet
• NECB GPP RE /- Flat
• NECB dC/dt (Chapin et al. 2006)
• (DPlant DHet DSOM)/Dt /- Flat

See Box 6.1
NEP and NECB (NBP at large scales) is most
relevant to long-term sequestration of CO2 from
atmosphere The problem of definition vs.
measurement
38
NEP is the difference between GPP and
Recosyst NEP NECB if lateral transfers are
small
Flat out dist., mig., leaching, sed., volatile
emissions, CH4
Flat in migration sediments dissolved C
6.8
39
NEP is the balance between two large fluxes GPP
and ecosystem respiration
6.9
40
A. Measuring NEP
41
Net ecosystem exchange
Chapin et al. 2006

• NEE net atmospheric CO2 flux

42
Measuring NEE - chambers
43
Measuring NEE Eddy covariance towers (eddy flux)
44
B. Controls on NEP, NEE, NECB

• 1. Represents net carbon storage in ecosystem
  (imbalance between C uptake and C loss)
• 2. Strong dependence on disturbance
• Negative when disturbance frequent (fire,
  tillage)
• Positive during recovery from disturbance
  (succession)

Schlesinger 2001
45
3. Biome differences in NEE reflect large net
carbon loss by respiration at high latitudes
6.10
Valentini
46
Why is NEP positive (NEE negative) in most
ecosystems?

• Maybe all ecosystems accumulate C between
  disturbances
• Maybe bias in site selection
• Researchers prefer productive sites?
• Maybe carbon loss by leaching is significant
• Maybe terrestrial biosphere is gaining carbon
• due to elevated CO2 and N deposition

47
Summary

1. Controls on NPP are similar to those on GPP.
2. Rplant consists of respiration for growth,
   maintenance, and ion uptake.
3. While variable temporally within ecosystems,
   across ecosystems NPP is 50 of GPP.
4. NEP, NECB reflect net storage of C within an
   ecosystem.
5. Disturbance regime is the main controller of
   difference between NEP and NECB in natural
   systems.
6. Humans are influencing many factors (temp,
   nutrient avail, disturbance regimes) that could
   alter the balance of GPP and Recosyst and thereby
   alter NEP and NECB.

48
Eddy flux advantages